Technology is constantly evolving, especially for printed circuit boards (PCBs). The aerospace industry, in particular, is always looking for PCBs with greater capabilities, including closer tolerances and better heat resistance. Design improvements such as more efficient layouts are critical for PCBs in aerospace applications.
Space shuttles and other vehicles use PCBs for various purposes, including communication, control, guidance, and navigation. The assembly and fabrication of PCBs also play a vital role in today’s aerospace vehicles and platforms.
PCBs in Aerospace
PCBs are commonly used in aerospace equipment, including space shuttles, planes, satellites, and communication systems. The requirements for PCBs used in these applications are similar to those in the automotive industry, although the operating conditions are even harsher. For example, aerospace PCBs must be made of materials that can withstand extreme vibrations and temperatures, among other environmental hazards. The need to operate in outer space for long periods also means these components must be highly reliable.
Specific applications for PCBs in aerospace include power supplies for systems such as aircraft, control towers, and satellites. PCBs are also essential in equipment accelerometers and pressure sensors, which pilots use to monitor aircraft performance. In addition, communications systems that maintain contact with ground control routinely use PCBs to ensure passenger safety.
PCB Aerospace Examples
Space shuttles like Discovery, Atlantis, and Endeavour are some of the best-known examples of aerospace applications for PCBs.
Discovery flew a total of 39 missions for the National Aeronautics and Space Administration (NASA) from 1984 to 2011, making it one of five shuttles to fly during this period. This shuttle used PCBs in electronic systems such as control and guidance. Atlantis flew a total of 33 missions from 1985 to 2011 and used PCBs in control, guidance, and navigation systems. Endeavour was the newest space shuttle, flying missions from 1992 to 2011. Its electronic systems made heavy use of PCBs, including communications and navigation.
Reliability of PCBs for Aerospace
Reliability is one of the most important factors for PCBs in aerospace applications, as they must perform in extreme environments. For example, satellites must transmit signals for long periods without failing due to the high cost of repair.
As a result, manufacturers must use strong substrates for aerospace PCBs and attach components very firmly to ensure the PCBs remain intact. PCBs are also subject to extreme vibration and shock in aerospace applications, so manufacturers must press component pins to the PCB rather than simply soldering them. In some cases, manufacturers use a combination of pins and soldering to keep components attached. In addition, PCBs in outer space must withstand high radiation levels, which will quickly damage normal equipment. This operating environment requires design changes such as anti-fuse technology.
The use of radio waves for communication in aerospace means that reliability is essential for this type of equipment. They require additional shielding in strategic locations on the PCBs and antennae. Shortening transmission lines can also increase the reliability of communications.
Corrosion is another issue that manufacturers should consider when making PCBs for aerospace applications. The relatively low cost of copper and high electrical conductivity make this metal a common material in PCSs. However, pure copper is also chemically active, requiring a coating over exposed copper to prevent corrosion. These coatings typically include aerosol coatings and solder masks, although manufacturers may also use epoxy coatings as a barrier to oxidation. In addition to protecting wiring from corrosion, these coatings must be highly heat-resistant.
PCB Assembly in Aerospace
PCB assembly for aerospace applications is the process of building and testing circuits on a PCB in an aerospace environment to ensure it works as intended. This process aims to identify and resolve any flaws with these systems before the PCB reaches the end user.
Aerospace PCB assembly offers several advantages over traditional PCB assembly methods. For example, it cuts parts and labor costs by eliminating the need to replace parts later in the assembly process. These PCBs function correctly from the first day of the assembly, a crucial requirement in aerospace. The production of quality parts also establishes a manufacturer’s reputation, leading to repeat business.
Aerospace PCB Assembly Requirements
The great importance of strength and reliability for PCBs in the aerospace industry means they need a high aspect ratio. This parameter describes the relationship between the size of the board and its circuits, which should be no more than 1:10. The general purpose of a PCB is to transfer electricity between its components, and one way to do that is to add more layers. This feature allows the PCB to handle more current, making it less likely to fail due to electrical overload.
Tolerance is another area of PCB design that manufacturers must consider, as there are several tolerance grades available. The two main types are commercial-grade and military-specification (mil-spec), with tolerance being the primary distinction between them. Commercial-grade boards have a five to ten percent tolerance, while mil-spec boards have tolerances below two percent. The boards used in aerospace are typical of mil-spec quality.
Aerospace-grade PCBs also have more significant requirements for the separation of components. This requirement primarily refers to separating ground and power planes and separating parts that generate high and low frequencies. The insufficient separation between these components increases noise and signal interference, which is particularly undesirable in aerospace applications.
High resistance to radiation is also essential for PCBs used in space, where there is no atmosphere to absorb high-energy particles that can damage electronic equipment. Radiation tolerance must therefore be an integral part of an aerospace PCB’s design.
Aerospace Materials
The extreme temperatures encountered by aerospace PCBs require them to be constructed of materials with a low coefficient of thermal expansion (CTE), meaning their size doesn’t change much in response to temperature changes. The laminate materials for these PCBs generally have a polyimide base that’s reinforced with glass. These materials have a CTE in out-of-plane directions of about 55 parts per million (ppm) per degree Celsius of temperature change (ppm/°C) and an in-plane CTE of about 15 ppm/°C. These thermal properties resemble ceramic-bodied microcircuits that manufacturers typically solder to a PCB.
This similarity in CTE reduces the stress on PCBs due to temperature changes because all the components are expanding and contracting to the same extent. As a result, solder joints can handle a larger number of thermal cycles, especially those involving large temperature changes. This improvement is crucial in aerospace environments, where PCBs must tolerate temperatures from -45°C to +85°C.
PCB Fabrication in Aerospace
The fabrication of aerospace PCBs is a distinctly different process from the assembly. While PCB fabrication transcribes a circuit board design onto the board’s physical structure, PCB assembly places those components onto the board.
Quality control and testing are vital phases in PCB fabrication due to the high requirement for reliability and safety. The PCBs used in aerospace are mission-critical components, so they need to perform flawlessly under extreme conditions. The fabrication of aerospace PCBs is thus subject to strict standards and regulations that help manufacturers avoid legal and financial consequences.
Quality control and testing also prevent the expense and delays of reworking PCBs by catching defects early in the fabrication process. Identifying and correcting problems early also reduces the risk of discarding the entire PCB, which would otherwise be a common occurrence in aerospace applications. Furthermore, quality control and testing ensure that a PCB’s behavior is consistent and repeatable, regardless of its complexity or batch size.
Design Considerations for Aerospace PCBs
The PCBs used in aerospace applications must operate in harsh conditions, including temperature extremes and exposure to corrosive chemicals. They must therefore meet IPC-A-610E Class 3 standards, which apply to high-performance electronic devices. This standard requires such devices to perform under unusually severe conditions without downtime. Thus, aerospace PCBs have more design considerations than standard PCBs operating in an office environment.
For example, all components of these PCBs must be mil-spec grade concerning tolerance and ability to handle the maximum current for that component. The low and high frequencies produced by the components should also be clearly differentiated. In particular, high-frequency components shouldn’t produce waveforms that interfere with the signals produced by low-frequency components. Furthermore, components that produce clock signals require additional shielding around them, typically forming an enclosure made of materials like aluminum.
Heat resistance is also a crucial design consideration, with common choices for metal components including AP, FR408, and Pyralux. Components that generate heat require additional clearance from other components, so designers must select an isolated location for hot components. Manufacturers should also use chemical compounds to dissipate heat from areas prone to overheating. They can identify these areas through pre-layout simulations to predict heat generation patterns when the PCB operates in a real-world environment.
Soldering requires additional care for PCBs used in aerospace applications. For example, manufacturers should pre-tin braided and stranded wires to make them easier to solder. They should also solder components to prevent vibrations from shaking them loose, even when press-fitted. In addition, manufacturers should recheck the thermal profiles of soldering processes before assembly to avoid damaging components during assembly.
The finishing material of aerospace PCBs should be able to tolerate harsh environments. The most common choices include:
- Electrolytic nickel and gold.
- Electrolytic wire bondable gold.
- Electroless nickel with immersion gold coating (ENIG) and immersion silver.
Hot air solder leveling (HASL) is a type of PCB finish that involves dipping the PCB into molten solder, which covers exposed copper surfaces. The PCB is then passed between hot air knives to remove the excess solder. Some government regulations prevent the use of lead in HASL solder.
Advancements for Aerospace Applications
The requirements of aerospace applications are driving innovation in PCB technology. These trends include integrated cores, miniaturization, and improvements in manufacturing processes.
Integrated Cores
The standard approach of using forced convection to dissipate may need to be revised for the higher operating temperatures of aerospace applications. Adding more layers of copper may be a practical solution on Earth, but the additional weight is a severe disadvantage in space. Recent solutions to this problem include creating very thin sheets of copper-graphite material, and replacing heavier layers of pure copper.
Developing multi-layered PCBs with an integrated thermal core is another new approach to design that provides a core with a CTE value closer to that of semiconducting materials such as silicon, gallium nitride (GaN), or silicon-carbon (SiC). This advance reduces the risk of temperature-induced stress, a common cause of failure in aerospace PCBs. However, it also requires manufacturers to make the core material in sizes that match existing standards for PCB dimension, which has proven a challenge given the thinness of these sheets.
Flight tests of PCBs with a copper-graphite core are underway, which may yield additional materials for aerospace applications. These advancements will benefit systems requiring effective thermal management, such as propulsion and high-power electronics.
Miniaturization
Advancements in PCB technology also drive miniaturization in electronics, which is vital for making aerospace equipment smaller and lighter. These developments will increase the fuel efficiency, range, and payload capacity of aerospace vehicles like aircraft and satellites. Advancements making PCBs smaller include high-density interconnects (HDI), which place components and traces closer together. Embedded components also reduce the size of PCBs, in addition to making them more reliable.
Placing more components on PCBs increases their functionality, allowing systems to reduce the number of PCBs they require. A multi-function PCB can often replace several traditional boards, reducing electronic systems’ weight and space requirements such as avionics, communication systems, and sensors.
Manufacturing Improvements
Improvements to manufacturing processes are another advancement that’s making PCBs more suitable for aerospace applications. For example, 3D printing, or additive manufacturing, is building a three-dimensional (3D) object from a digital 3D model. Various techniques exist, but they generally involve using a computer to add material in layers, whether by deposition, joining, or solidification. Manufacturers can use 3D printing to construct plastics, liquid, or powder objects.
In the case of PCBs, 3D printing reduces manufacturing times and costs while also increasing the customization and flexibility of design. As a result, rapid prototyping is more accessible, which is particularly beneficial for developing PCBs for aerospace applications.
Work with a Certified Aerospace PCB Partner
The aerospace industry increasingly depends on using advanced electronics in its vehicles and platforms. This industry has quality requirements beyond the typical office environment where standard PCBs typically operate. They must tolerate environmental extremes that would quickly render a standard unit inoperative. Furthermore, repairing or replacing parts in space is extremely expensive, especially for uncrewed vehicles. In addition, size and weight are critical design considerations for all equipment in aerospace applications.
As a AS9100D/ISO9001:2015 Certified PCB partner, Imagineering Inc. understands the growing reliance on advanced electronics in the aerospace industry. We recognize the need for reliable and durable PCB solutions with heavy consideration around size and weight. Elevate your aerospace electronics to new heights by leveraging the experts at Imagineering Inc. Get a quote today!